hhGGhh Welcome to the community!
I recommend you to contact the company R&M, which made the Installation at home. You should install a junction box in the distribution box. Then you can plug in connector A on the modem and the other end piece into the box installed by the company. Then you feed all the RJ45 cables into the appropriate rooms, which corresponds to the point in picture D. This way it should work fine.
Greetings
Daniele